One of the greatest problems since the inception of lithographic offset printing has been to achieve the application of moistening or dampening fluid to the surface of a lithographic printing plate in uniform and evenly distributed quantities and in regulated amounts so as to assure a uniformly good reproduction of a printed image on paper. As is well-known, a lithographic offset printing plate is chemically treated to provide an ink receptive printing area and a hydrophylic or moisture receptive non-printing area. The film of dampening fluid which is applied to the surface of the plate is retained by the hydrophylic area where ink is rejected. The printing area receives both water and ink. Therefore, only the image of the printing area is transferred to the blanket cylinder of the lithographic printing press and from there to the paper on which the image is printed.
In the past, an early dampening system for a lithographic printing press consisted of a pan of water in which a metallic pan roller rotated. An oscillating ducter roller alternatively rotated in contact with the pan roller and a vibrating metal roller which in turn was in rotative contact with one or more fabric-covered rollers that contacted the printing plate. The moisture that was transferred from the fabric-covered rollers to the printing plate was dependent to a great extent upon the moisture absorbed in the fabric covering material. With such dampening systems, the plate is subject to frictional wear due to contact with the fabric-covered dampening rollers, and the fabric soon became impregnated and contaminated by the ink which accumulated thereon from continued exposure to the image area of the printing plate. To compensate for this condition, additional water was fed to the ink impregnated areas of the dampening rollers, but this caused an excessive amount of moisture to be directed to the less ink impregnated areas of the rollers.
In an attempt to solve the problems presented by the fabric-covered dampening rollers, a continuous-duty, plate contacting, inker feed dampening system was developed to eliminate the fabric-covered dampening rollers. Such inker feed dampening systems generally include a pan roller which may be a metallic or resilient-covered roller rotatably disposed in a pan of dampening fluid. A resilient non-absorbent metering roller is impressed against, and is in rotative contact with a hydrophylic transfer roller so as to transfer dampening fluid to the hydrophylic surface of the transfer roller. The transfer roller is preferably chrome-plated and has an exterior surface which is highly polished and treated so as to render the same moisture receptive or hydrophylic. Often this transfer roller is a chemically etched roller which rotates in contact with an ink form roller. Dampening solution is metered into a relatively thin film by the metering nip formed between the chrome transfer roller and the resilient metering roller, and this metered film of dampening solution moves to a nip between a first ink form roller and the transfer roller so as to then be transferred to the plate cylinder. The transfer roller and the metering roller are positively driven at a slower speed than the form roller so that a "slip nip" is formed at the juncture of the transfer roller and an ink-coated applicator roller. Such an inker feed dampening system is illustrated by U.S. Pat. No. 3,168,037 to H. P. Dahlgren.
Continuous-duty, plate-contacting, inker feed dampening systems generally provide a relatively fast response time at the beginning and during a printing run with controlled water in ink emulsification, so as to limit the number of sheets of paper which are wasted to provide acceptable printed copies. However, such dampening systems normally require a water soluble organic liquid, such as isoprophyl alcohol, to be added to or used as the dampening fluid to reduce the surface tension of the water in the fluid and to prevent a water film from collecting ink globules on the surface of the form roller from the hydrophylic chrome transfer roller.
The control of dampening fluid supplied by an inker feed dampening system is achieved by separate motor drives which drive at least one of the dampening system rollers at a slower speed than the plate cylinder, while other rollers in the dampening system run at essentially the same surface speed as the plate cylinder. Additional adjustment of the dampening fluid supply is obtained by varying the pressure (stripe) between dampening system rollers and sometimes by varying press speed.
Many modifications have been made to the typical continuous-duty inker feed dampening system in an attempt to rectify such problems as ghosting and ink emulsification. A chrome roller has been run at the same speed as the dampener form roller while a resilient roller has been placed to function as a transfer roller while being driven by a variable speed motor. A second chrome roller is placed in the pan against the resilient rollerand in this system, only the dampening form roller carries ink.
In another modification, a separate dampening form roller has been employed in contact with the chrome transfer roller and a bridge roller is used to connect the dampening form roller to the first ink form roller for the system. This basic configuration has been further modified by positively driving the separate dampener form roller and the bridge roller at a speed different from that of the plate cylinder. Such a system is shown by U.S. Pat. No. 4,724,764 to J. McPhee et.al.
Prior dampening systems of the type described which incorporate "slip nips" between various system rollers require adjustment of roller settings and/or speeds to obtain a desired ink/water balance. These settings or speeds must also be adjusted as press speeds vary, or with a variation in printing plates, inks, paper, fountain solutions, and the other variables. Also, as previously indicated, when chrome transfer rollers are used in contact with an ink form roller, the use of a dampening fluid containing a water soluble organic liquid is required.